1. Technical Field of the Invention
The present invention is directed to an optical fiber bundle, and more specifically, to an assembly comprising an optical fiber bundle spliced to a single optical fiber, and method of manufacturing the assembly.
2. Description of the Related Art
A major issue in high power fiber amplifiers and lasers is how to couple significant optical power from pump diodes into double clad rare earth doped optical fibers, while maintaining access to the input and output of the fiber amplifiers and lasers. US20030202547A1, entitled “Multi-mode fiber amplifier”, and hereby incorporated by reference herein, represents an example of this class of devices. This patent describes single mode propagation in multimode fiber, and references such techniques as mode-filtering, the use of few moded fiber, gain-guiding, and other techniques which may find general applicability in connection with the invention.
U.S. Pat. No. 5,854,865 to Goldberg et al. discloses an end-pumping scheme where a high power pump is coupled into the double clad fiber end through beam shaping bulk optics. U.S. Pat. No. 6,529,657 B2 to Goldberg et al. discloses a side-coupling scheme using a polished V-groove at one side of the double clad fiber, along with bulk beam shaping optics. These two approaches are based on very high power pump diodes for very high power fiber lasers, the reliability of which is not yet fully proven. Although theoretically it is possible to scale up these two schemes, in practice, it is very difficult to accomplish.
A number of fiber based solutions have also been proposed. U.S. Pat. No. 5,999,673 to Kapontsev et al. discloses tapering only the multimode pump fiber, and then fusing it to the double clad fiber from the side. U.S. Pat. No. 5,864,644 to DiGiovanni et al. discloses a large number of multimode pump fiber pigtails fused with a single mode fiber in the center and then stretched down to a small diameter. The taper waist is then cut and spliced to the double clad fiber. U.S. Pat. No. 6,434,302 B1 to Fidric et al. discloses that a fused fiber taper is cut beyond the waist, so that the fiber tapers to the small waist and then increases in diameter to a fiber end, which is subsequently spliced to the double clad fiber. This fiber coupler based approach is easy to scale up. Coupling ratios higher than 60:1 has been demonstrated. This allows much more reliable lower power pump diodes to be used to achieve high average power. The elimination of the beam shaping bulk optics and end face where optical power is very large also simplifies the fiber laser structure, and consequently improves its reliability and manufacturability.
As described above, fiber-based approaches are more desirable, due to higher reliability, stability, scalability and manufacturability. However, production of these prior art pump couplers involves complex manufacturing techniques, entailing pulling a few tens of fibers at the same time by highly precise motion controls in a very controlled manner, while the fibers are heated. All the fibers involved have to be tapered adiabatically to avoid major transmission losses. This requires that the heating and stretching of each fiber be well controlled. This gets more difficult as this process scales up to more and more fibers.